This invention generally relates to an apparatus and method for the measurement of electrical energy consumption for revenue metering and various other applications.
In the past electronic kilowatt-hour meters used in revenue metering were discrete instruments that utilized a current transformer ("CT") with the meter to which it was connected being typically installed on the wall in a utility room. Wiring had to be installed between the meter and the measurement point. CTs, used to inductively sense the amount of current flowing through a loaded conductor, were installed in or near an adjacent electrical box thus requiring additional external wiring.
A CT used in revenue metering applications typically comprises a one-piece toroidal coil, usually referred to as a "doughnut", which is mounted around a loaded conductor to be measured. Since the doughnut style CT does not come apart, in order to install it around a wire, the wire had to be disconnected so that it could be installed through the hole in the doughnut. Connecting a wire then, required that the electrical power had to be interrupted for the period of time while installation was taking place. In some applications this is a major problem, especially where uninterruptable loads, such as computers and alarm systems are involved. Additionally, such an interruption can be very costly especially where a lot of users are affected by the installation.
Split-core type current transformers have been available for some time and are not novel in and of themselves. Such a design enables split sections of a CT to be temporarily disengaged to allow a loaded conductor to be surrounded by the CT when it is re-engaged. This is manifested in pincher-type pickups which are typically used in connection with hand-held ammeters. Inexpensive split-core CT designs generally employ core halves which merely butt together when engaged. At most, there was minimal overlap between split-core portions. As a result, the split-core CT's in the prior art are not accurate enough for revenue metering since the inductive coupling of the split-core CT design tended to be much more inaccurate than their doughnut-style counterpart.
Additionally 3-phase meters basically required 3 single-phase meters to be integrated into one 3-phase meter. That meant a lot of models of dedicated meters had to be developed to meet all of the various applications, such as 3-phase, 3-element meters, 3-phase, 2-element meters, 1-phase, 1-element and 1-phase, 2-element meters for various power applications. This required discrete and separate designs.
What was needed was an accurate, split-core current transformer that could be easily and inexpensively installed. Additionally, it was desirable to have a compact CT with an integrated meter which could be combined in multiple-phase metering applications and which did not require a lot of extraneous wiring.